1. Field of the Invention
The invention relates to an emission device for an ultra-high pressure mercury lamp. The invention relates especially to an emission device which is used for a liquid crystal display device with a light source which is a short arc ultra-high pressure mercury lamp in which an arc tube is filled with at least 0.15 mg/mm3 of mercury, and in which the mercury vapor pressure during operation is at least 110 atm, and for a projector device such as a DLP (digital light processor) or the like using a DMD (digital micro mirror device).
2. Description of the Prior Art
In a display device of the projection type, there is a demand for illumination onto an image device in a uniform manner, and moreover, with adequate color rendering. Therefore, the light source is a metal halide lamp which is filled with mercury and a metal halide. Furthermore, recently smaller and smaller metal halide lamps and more and more often point light sources are being produced and metal halide lamps with extremely small distances between the electrodes are being used in practice.
Against this background, instead of metal halide lamps, lamps with an extremely high mercury vapor pressure, for example, with 150 atm, have been recently proposed. Here, the broadening of the arc is suppressed (the arc is compressed) by the increase of the mercury vapor pressure and moreover a great increase of the light intensity is the goal.
These ultra-high pressure discharge lamps are disclosed, for example, in Japanese patent disclosure document HEI 2-148561 (U.S. Pat. No. 5,109,181) and Japanese patent disclosure document HEI 6-52830 (U.S. Pat. No. 5,497,049).
For the above described lamp, for example, a short arc ultra-high pressure mercury lamp is used in which in the silica glass arc tube there is a pair of opposed electrodes with tungsten as the main component and spaced apart by a distance of at most 2 mm. This arc tube is filled with at least 0.15 mg/mm3 of mercury and also contains an amount of halogen in the range from 1×10−6 μmole/mm3 to 1×10−2 μmole/mm3. The main purpose of adding the halogen is to prevent devitrification of the arc tube. However, the so-called halogen cycle also arises as a result.
In the above described ultra-high pressure mercury lamp (hereinafter also called only a “discharge lamp”) the phenomenon occurs that, in the course of operation, tungsten is deposited on the electrode tips, that the electrode tips deform and that projections form. This phenomenon is not entirely clear, but the following can be assumed.
The tungsten which is vaporized from the high temperature area in the vicinity of the electrode tip during lamp operation combines with the halogen and residual oxygen which are present in the arc tube. In the case, for example, that bromine (Br) is added as the halogen, it is in the form of a tungsten compound such as WBr, WBr2, WO, WO2, WO2Br, WO2Br2 or the like. These compounds decompose in the high temperature area in the gaseous phase in the vicinity of the electrode tips and yield tungsten atoms or tungsten cations. Due to thermal diffusion (diffusion of the tungsten atoms which are moving from the high temperature area in the gaseous phase (=arc center) in the direction toward the low temperature area (=vicinity of the electrode tip)), due to concentration diffusion and due to the fact that in the arc the tungsten atoms are ionized, yield cations and during operation of the electrode as a cathode are pulled (drift) by the electric field in the direction to the cathode, the tungsten vapor density in the gaseous phase in the vicinity of the electrode tip becomes high, by which deposition on the electrode tip takes place and the electrode tips deform, or by which projections are formed. The circumstances of the above described projections are disclosed, for example, in Japanese patent disclosure document 2001-312997 (U.S. Pat. No. 6,545,430).
FIG. 9 schematically shows the electrode tips and projections. In the figure, the each of the electrodes 1 has a spherical part 1a and a shaft 1b. On the tip of the spherical part 1a, a projection 2 is formed. In the case in which, at the start of lamp operation, there is no projection, projections 2 are also produced by subsequent operation, as is shown in FIG. 9. An arc discharge A is formed on these projections 2 as the starting points.
In this case, if the above described projections are formed in ideal form, there is no disadvantage. However, since they form, as was described above, in the course of operation of the discharge lamp by a physical phenomenon, they do not always have an ideal shape. Especially in the case in which they have a crooked shape, such as showing asperity, the starting point of the discharge becomes unstable, which causes the serious disadvantage that, in the case of a light source of a projector device which is designed such that a point light source is assumed, light cannot emerge to a sufficient degree. Similar disadvantages also arise.